Recently, various methods have been developed to improve the utilization efficiency of frequency bands and thereby achieve high-speed data communication in a digital radio communication system. For instance, methods of executing multi-valued digital modulation/demodulation according to modulation methods using phase information for data decision, such as quadrature amplitude modulation (QAM) and phase shift keying (PSK), have been known. In a modulation method using phase information for data decision, phase errors (phase noise) that occur in a transmitter and a receiver may cause degradation of a bit error rate (BER). A receiver using this modulation method may improve its bit error rate by performing phase error compensation (phase noise compensation).
FIG. 16 is a block diagram showing the constitution of a demodulation circuit 9 which is used in a receiver of a digital radio communication system.
The demodulation circuit 9 includes a reference oscillator 1001, a quadrature detector 1002, an A/D converter 1003, and a carrier recovery circuit 91. The carrier recovery circuit 91 includes a phase rotator 1004, a phase error detector 1005, a loop filter 1008, and a numerical control oscillator 1009. The carrier recovery circuit 91 configures a carrier recovery loop equivalent to a phase-locked loop (PLL).
The reference oscillator 1001 produces a reference signal having a fixed frequency. The quadrature detector 1002 performs quadrature detection on an IF input signal r91 (where IF stands for Intermediate Frequency) by use of the reference signal of the reference oscillator 1001, thus producing an in-phase channel (Ich) baseband signal and a quadrature-phase channel (Qch) baseband signal. The quadrature detector 1002 sends these baseband signals to the A/D converter 1003. Baseband signals of the quadrature detector 1002 may contain phase errors due to a phase difference between the intermediate frequency (IF) and the fixed frequency of the reference oscillator 1001. The A/D converter 1003 performs analog/digital conversion on baseband signals of the quadrature detector 1002 so as to send them to the phase rotator 1004.
The phase rotator 1004 performs phase error compensation by way of phase rotation on digital baseband signals from the A/D converter 1003, thus producing output signals r92 ascribed to an in-phase channel (Ich) and a quadrature channel (Qch). The phase error detector 1005 detects phase errors, which remain in Ich/Qch output signals r92, so as to produce a phase error signal representing a voltage value equivalent to detected phase errors. The loop filter 1008 eliminates unnecessary high-frequency components included in a phase error signal. The numerical control oscillator 1009 produces a sine-wave signal and a cosine-wave signal with a phase inverse to a phase indicated by a phase error signal that has passed through the loop filter 1008, thus sending them to the phase rotator 1004. The phase rotator 1004 performs phase error compensation on baseband signals based on a sine-wave signal and a cosine-wave signal from the numerical control oscillator 1009.
Since the loop filter 1008 eliminates unnecessary high-frequency components included in a phase error signal of the phase error detector 1005, it is possible to suppress short-term fluctuations of a sine-wave signal and a cosine-wave signal of the numerical control oscillator 1009 by way of the phase error compensation of the phase rotator 1004. That is, it is possible to stabilize the operation of the carrier recovery circuit 91 by way of a PLL loop. Herein, the loop filter 1008 needs to adopt optimum bandwidths which differ from each other depending on the magnitude of phase errors included in baseband signals of the quadrature detector 1002 and their modulation methods. For this reason, it is preferable to adjust the bandwidth of the loop filter 1008 based on the magnitude of phase errors and its modulation method.
Various methods have been known as methods for adjusting the bandwidth of the loop filter 1008. For instance, PLT 1 disclosed a digital satellite broadcasting transmitter/receiver, in which an error correction circuit measures a bit error rate so that a control circuit sets a loop filter coefficient based on a decision as to whether or not the bit error rate is higher than a predetermined threshold. PLT 2 disclosed a QAM carrier recovery circuit in which, upon estimating phase noise and additive noise, a user is able to optimize a loop bandwidth of a carrier wave based on the estimation result. PLT 3 disclosed a carrier recovery device that calculates phase errors based on coordinates of signal points (or constellation points) and coordinates of demodulated signals in a signal-point alignment (or a constellation) according to QAM or PSK. PLT 4 disclosed a carrier recovery circuit that determines phase lag or phase lead in carrier recovery by dividing a peripheral area, encompassing constellation points in a constellation of QAM, into four subdivisions.